The invention relates to a method of controlling the bitrate of an assembly of video encoders of the MPEG type each processing, in parallel, an individual video signal, the pictures to be encoded being of a different type T referred to as I, P, or B, respectively, dependent on whether they are submitted to an intraframe encoding mode independent of any other picture, or to a predictive encoding mode employing a unidirectional motion prediction based on another intraframe-encoded or predicted picture, or to a bidirectional interpolation encoding mode based on a previous picture and a subsequent picture.
The invention also relates to a device for carrying out said bitrate control method.
It is known that the encoded pictures of different sequences have equivalent qualities when they are encoded using the same quantization step before correction due to adaptive spatial activity (by means of quantization), and that the encoders are not submitted to any restrictions. That is, the structures of the groups of pictures, or GOPs, may be different and picture synchronism between the encoders is not necessary. The MPEG standard defined by the International Standardization Organization ISO is described, for example in the document "MPEG: A video compression standard for multimedia applications", D. Le Gall, Communications of the ACM, April 1991, Vol. 34, No. 4, pp. 46-58. In an assembly of encoders of the MPEG type, as in the present application, each encoder has an output buffer memory or buffer (a buffer is a memory which receives bits at a variable bitrate and restores them to a constant bitrate, or conversely in the case of decoding). The global contents of a global buffer corresponding to the sum of the contents of the individual buffers constitute an indicator with which the encoding bitrate can be controlled by acting on the quantization step. The fluctuations of the global buffer contents, which are caused by the difference between the instantaneous encoding bitrate and the transmission bitrate, depend on the quantization step and on the type of picture being encoded at any instant (pictures of the type I, P, B referred to as intraframe-predicted, or interpolated pictures in accordance with the encoding mode as indicated above). These considerable bitrate fluctuations also occur in the buffers of the encoders and in the similarly provided buffers of the decoder after the transmission and/or storage of the encoded signals, and give rise to a considerable delay between encoding and decoding of a picture.